Photosensitive elements which can be used in image reproduction processes are well known in the graphics arts industry. Such elements which may be positive or negative-working elements usually comprise a support, a photosensitive layer and a cover sheet. The photosensitive layer is generally tacky prior to exposure. The positive-working photopolymerizable elements are exposed imagewise through an image bearing transparency to form an image having tacky and non-tacky areas. Negative-working photosensitive elements are also known which require a dual exposure to provide a negative image with tacky and non-tacky areas.
Such photosensitive elements are widely used in off-press color proofing to simulate the images produced by printing. In a surprint proof, all the colored images are superimposed, by, for example, multiple exposure, lamination or transfer, onto a single support. Unlike an overlay proof, the colored images cannot be separated and viewed individually.
Chu and Cohen, U.S. Pat. No. 3,649,268 describe a reproduction process using positive-working photopolymerizable elements. The image is developed by toning with a suitable toner which desirably adheres only to the tacky image areas. Excess toner which may be present is removed from the non-tacky areas to provide, for example, an image which is a proof of the original or which can be used to transfer the image to another surface. Multicolor proofs can be made as well.
The photosensitive elements used to make multicolor proofs are generally in roll form. In making a four color proof, comprising cyan, magenta, yellow and black, four different roll laminators are required for the lamination step. This adds cost to the process. If a sheet laminator could be used, only one laminator would be required to make a four color proof. The problem with some known photosensitive elements is that once the cover sheet is removed exposing the tacky photosensitive layer, the photosensitive layer on the support cannot be fed through a sheet laminator.
A need exists for a photosensitive element that has a surface that is non-tacky at room temperature and becomes tacky or softenable upon the application of heat. These elements would be capable of being fed through a sheet laminator eliminating the need for four separate laminators and thus reducing the cost of making a four color proof. Also, the element may be rolled into a roll without the need for a cover sheet further reducing costs.